Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Fix typos.

PR: bin/148894
Submitted by: olgeni

Don't expose the uart_ops structure directly, but instead have
it obtained through the uart_class structure. This allows us
to declare the uart_class structure as weak and as such allows
us to reference it even when it's not compiled-in.
It also allows is to get the uart_ops structure by name, which
makes it possible to implement the dt tag handling in uart_getenv().
The side-effect of all this is that we're using the uart_class
structure more consistently which means that we now also have
access to the size of the bus space block needed by the hardware
when we map the bus space, eliminating any hardcoding.

Make uart_getenv() not be ns8250 dependent. This will allow, in the future,
compilation of kernels without ns8250 support but using the uart framework.
These kernels will be for machines where size matters more, so including code
that can never be executed is undesriable...

Start each of the license/copyright comments with /*-, minor shuffle of lines

Make sure the baudrate specified with the BR tag is somewhat sane.
A baudrate we consider insane is silently replaced with 0. When the
baudrate is 0, we will not try to program the hardware. Instead we
leave the communication speed unaltered, maximizing the chance to
have a working console. Obviously this means we allow specifying a
0 baudrate for exactly that purpose.

Fix braino in previous commit: getenv() can return NULL.

Introduce the hw.uart.console and hw.uart.dbgport environment variables
to select a serial console and debug port (resp). On ia64 these replace
the use of hints completely and take precedence over hints on alpha,
amd64 and i386. On sparc64 these variables are not yet recognised.

The reasons for introducing these variables are:
1. Hints have side-effects. They reserve the unit number for use by
isa or acpi devices and therefore cannot be used to select a pci
device. Also, the use of a unit number to select a device prior
to bus enumeration is nonsense. The new variables have no side-
effects and are not based on unit numbers.
2. Hints don't have the expression power to allow the sysadmin to
select UARTs that are not legacy PC devices and need the support
of compile-time constants to give the sysadmin some level of
flexibility.

The hw.uart.console and hw.uart.dbgport variables specify a list of
attributes. An attribute is a tag-value pair, seperated by a colon.
Attributes are seperated by a comma. Where possible, tags are the
same as those in /etc/remote (only br and pa in practice). Details
can be found in the manpage (not part of this commit).

Not tested on: amd64, pc98